The transport properties of perfluorinated cation exchange membranes in chlor‐alkali cell solution environments have been studied using a newly designed membrane test cell. A radiotracer‐weight method was used to measure sodium ion and water transport numbers for two Nafion® perfluorosulfonate membranes. A homogeneous film shows rapidly decreasing sodium ion transport numbers with increasing Formula catholyte concentration, reaching a value of 0.56 mol F−1 with 12.5M solution at 80°C. A fabric‐backed material with a sulfonamide exchange site layer yields considerably higher values with a more complicated concentration dependence. Water transport numbers for the latter membrane are also higher; a value of about 3 mol F−1 is found for typical experiments in which the anolyte solution is Formula . Results are discussed in terms of the mechanism of ionic membrane transport in relation to polymer water and electrolyte sorption.